Why is Mars so much Smaller than Earth?

Solar system and planetary formation is a common field of study for astrophysicists and planetary scientists, but why is Mars so much smaller than Earth?

Mars has only 10 percent of the ​mass of the Earth, but current, and conventional, planetary formation models predict Mars should be much larger. It should be similar in size to Earth and Venus.

Possibly even larger.

So why isn't it?

It's a puzzle. ​

In the standard model of planet formation, similarly sized objects accumulate through a process called accretion; rocks collide and merge with other rocks, creating mountains; then mountains merge to form bigger mountains, and so on.​

For decades, this model has worked fairly well. It's good at predicting the size Earth and Venus -- but it also predicts that Mars should be of similar size, or even larger than Earth. And, on top of that, the model predicts that the asteroid belt should be much more massive than it is.

Using a new process in planetary formation modeling, where planets grow from tiny bodies called "pebbles," SRI scientists think they can explain why Mars is so much smaller than Earth. The same process also explains the rapid formation of the gas giants Jupiter and Saturn.

​In describing the new model, Hal Levison, an Institute scientist at the SwRI Planetary Science Directorate and lead author of a new paper published in the Proceedings of the National Academy of Sciences of the United States (PNAS) Early Edition, said:

Understanding why Mars is smaller than expected has been a major problem that has frustrated our modeling efforts for several decades. Here, we have a solution that arises directly from the planet formation process itself. This new simulation actually reproduces the structure of the inner solar system, with Earth, Venus, and a smaller Mars.

In fact, the new model clearly demonstrates the structure of the inner solar system is actually the result of a growth process known as Viscously Stirred Pebble Accretion (VSPA). With VSPA, dust readily grows to "pebbles" - objects a few inches in diameter - some of which collapse to form asteroid-sized objects.

Under the right conditions, these asteroids can 'feed' on the remaining pebbles -- aerodynamic drag pulls pebbles into orbit where they then spiral down and fuse with the growing planetary body. The process means certain asteroids can become planet-sized over relatively short time scales.

The new model, however, finds that not all of the asteroids in the early Solar System were well-positioned to accrete pebbles and grow.

Take Ceres, for example. About 600 miles across, Ceres is the largest asteroid in the asteroid belt. In this new planetary formation model, it would have grown very quickly if it had formed near the current location of the Earth. But farther out from the Sun, it would not have been able to grow as effectively because the aerodynamic drag would have been too weak for pebble capture to occur -- which also explains why Mars is so much smaller than Earth.

As Levison's co-author Katherine Kretke​ explained:

This means that very few pebbles collide with objects near the current location of Mars. That provides a natural explanation for why it is so small. Similarly, even fewer hit objects in the asteroid belt, keeping its net mass small as well. The only place that growth was efficient was near the current location of Earth and Venus.

Bill Bottke, another co--author of the paper, added:​

This model has huge implications for the history of the asteroid belt. Previous models have predicted that the belt originally contained a couple of Earth-masses' worth of material, meaning that planets began to grow there. The new model predicts that the asteroid belt never contained much mass in bodies like the currently observed asteroids.

An earlier paper by Levison, Kretke, and Martin Duncan (Queen's University) demonstrated that pebbles can form the cores of the giant planets and explain the structure of the outer solar system. But this model extends that work into the inner Solar System -- and explains why Mars is so much smaller than Earth.

And combined, the two studies explain how the entire solar system was formed from a single, unifying process -- and overcome the gaps in previous models.

As Levison concluded:​

As far as I know, this is the first model to reproduce the structure of the solar system - Earth and Venus, a small Mars, a low-mass asteroid belt, two gas giants, two ice giants (Uranus and Neptune), and a pristine Kuiper Belt.